Acetone and butanol are excellent organic solvents and important chemical materials. Butanol can be used as a solvent for paint and surface coating materials, as well as for the production of plastic and rubber products. Moreover, butanol can also be utilized to produce chemical products such as butyl acetate, butyl acrylate, butyraldehyde, butyric acid, butyl amine and butyl lactate, etc. Butanol is an attractive liquid fuel, other than ethanol. Acetone is mainly used as the solvent for producing cellulose acetate films, plastics and coating materials. Besides being used as a solvent, acetone can also be mainly used to produce chemical products, such as methyl methacrylate (MMA), bisphenol A, aldol condensation substances, and the like. Acetone-butanol-ethanol (ABE) fermentation is a traditional large-scale fermentation process, which, as merely inferior to ethanol fermentation, has been the second most important fermentation process in the world. Since the early stage of PRC, corn flour has been used for the industrial production of ABE fermentation, so that a reliable fermentation technique had been built up. Then the ABE fermentation technique declined because of the development of petrochemical industry. However, due to the increasing risks in environmental issues such as the exhaustion of petrochemical resources, greenhouse effects and the like, production of chemical materials and energy substances from renewable resources has attracted more and more attention. Therefore, ABE fermentation, once again, exhibits its advantages in this competition. Nevertheless, the total concentration of the solvents obtained using the current ABE fermentation process is very low. In general, the total concentration of the solvents (acetone, butanol and ethanol) ranges approximately from 15 to 20 g/L. As a result, the cost for extracting and separating the solvents is very high, which restricts the application of acetone and butanol as well as the production of ABE fermentation. Ever since the production of acetone and butanol by fermentation became possible, researchers had been working intensively to develop a convenient, low-cost and efficient separation method.
The traditional process for the purification of acetone-butanol-ethanol involves the following steps. First, a fermentation broth is distilled and concentrated by passing through a simple distillation column, so that the solid impurities and partial water are removed from the fermentation broth and 4% of ethanol, 10% of acetone, 26% of n-butanol and 60% of water are obtained. Then distillation is carried out again to further purify the solvents, i.e. ethanol, acetone and n-butanol. A huge amount of energy is consumed in this process. In fact, about 18 tons of steam is required to produce 1 ton of solvents in industry (Chen, T. Chemical Industry Press, 1991). Especially, approximately 10 tons of steam is consumed in the separation process, which accounts for 60% of total energy consumption. Presently, the major technologies used to extract and separate acetone and butanol from fermentation broths include adsorption, gas stripping, liquid-liquid extraction, pervaporation, repulsive extraction (salting-out) and so on. In general three different types of materials including diatomite, activated carbon and polyvinylpyrrolidone were used as absorbents in adsorption technology. Meagher et al. (U.S. Pat. No. 5,755,967[P], 1998) utilized diatomite to absorb acetone and butanol from the fermentation broths, and found that that diatomite had a higher ability for absorbing butanol and acetone. Unfortunately, desorption was not investigated in this study. Compared with other methods, adsorption technology has some disadvantages, including the higher cost, complicated manipulation, low selectivity, high energy consumption and liableness to the contamination caused by fermentation broths (Biotechnology Advances, 2000, 18(7): 581-599). Coupling between gas stripping and ABE fermentation can increase the fermentation yield and substrate utilization. However, this technology relies on many factors, such as the recovery speed of carried gas, bubble size, antifoaming agents and so on, and thus involves a complicated operation procedure. For liquid-liquid extraction, most researches concerning the extractants focused on oleyl alcohol (Current Opinion in Biotechnology, 2007, 18: 220-227), benzyl benzoate, dibutyl phthalate (Journal of Fermentation and Bioengineering, 1995, 2(80): 185-189), biodiesel (Chinese Journal of Bioprocess Engineering, 2007, 5(1): 27-33) and so on. However, liquid-liquid extraction also has some disadvantages, such as its low distribution coefficient, low recovery yield, occurrence of emulsification and so on. Luo et al. utilized pervaporation membrane separation technology (Chemical Engineering (China), 2010, 38(2): 43-46) to study the separation of ABE system. The separation was started from a simulated fermentation broth. The results indicated that pervaporation technology can be used as an efficient method for the separation and concentration of butanol from the ABE system, but is not suitable for efficiently separating acetone from the ABE system. Moreover, pervaporation strongly relies on the properties of the membrane materials. The separation performance and flux depend on the properties of the membrane to a great extent. The short lifetime of the membrane restricts the application of this technique to some extent (Appl. Microbiol. Biotechnol., 1998, 49: 639-648). Hu et al. utilized repulsive extraction to study the effect of various salts on the separation of acetone and butanol from a simulated ABE fermentation broth. The results indicated that utilization of a composite extractant can greatly increase the distribution coefficient and selection coefficient of butanol and acetone in the two phases, and thus, to some extent, accomplishes the separation and concentration of butanol and acetone (Journal of South China University of Technology, 2003, 31:58-62).
Recently, while investigating solvent crystallization, some researchers found that at a proper temperature, liquid-liquid phase separation, rather than salt crystallization, may occur if the concentrations of the inorganic salts, organic solvents and water are appropriate in the system. Moreover, if hydrophilic low-molecular substances, such as methanol, ethanol, acetone and so on, were used as the organic solvents, a novel aqueous two-phase extraction system could be formed. Compared with the traditional aqueous two-phase system composed of high molecular polymer and salt, this novel aqueous two-phase system has many advantages, for example, the phase separation is clearer; the cost is lower; and no polymer having a high viscosity and disposal difficulties exists in the extraction phase. Although the researches focusing on this field has just started in the whole world, its excellent separation performance has been noticed. For example, the dipotassium hydrogen phosphate/ethanol system was used by Louwrier to extract biomacromolecules, such as bovine serum albumin (BSA), α-casein, ribonuclease and so on (Biotechnology Techniques, 1998, 12 (5): 363-365). The acetone/sodium chloride system was utilized by Li and Gao et al. to extract metal complex and metal ion (Chinese Journal of Applied Chemistry, 2001, 18 (3): 241-243; Journal of Instrumental Analysis, 2002, 21 (3): 75-77). All of the above researches have achieved satisfying results. This novel aqueous two-phase system was also used to separate 1,3-propanediol and 2,3-butanediol from fermentation broths in our previous work. The separation of the target products from fermentation broths was accomplished effectively, and the separation effect was prominent (The Chinese Journal of Process Engineering, 2008, 8 (5): 888-900; Process Biochemistry, 2009, 44, 112-117; Biotechnol. Lett, 2009, 31 (3): 371-376; Separation and Purification Technology, 2009, 66: 472-478). In fact, salting-out effect not only benefits the extraction of hydrophilic organic solvents, but also improves the traditional organic extraction. The combination of salting-out and extraction forms a novel salting-out extraction (SOE) technology, and the novel aqueous two-phase extraction is one type of the SOE technology. Up to now, SOE systems have not been reported to be used in the separation of the acetone-butanol-water system. Compared with the traditional extraction using organic solvents, SOE technology has many advantages, including the high distribution coefficient, high recovery yield and low solvent consumption, etc. Compared with the traditional salting-out or repulsive extraction, SOE technology involves low salt consumption and low corrosiveness to equipments. Moreover, the inorganic and organic salts can be recycled and used for multistage salting-out extraction. Compared with the traditional aqueous two-phase PEG/inorganic salt system, SOE technology has advantages such as high distribution coefficient, rapid phase separation, easy recycle of solvents and products, low cost and so on. Compared with other separation methods, SOE technology also has some advantages, including its simple operation procedure, low energy consumption and high efficiency. Furthermore, SOE technology allows the separation of acetone and butanol directly from a fermentation broth, i.e. the step of solid-liquid separation is omitted.